223. Weather_ Tornado Science, in a Land With Plenty of Experience pptx

Weather: Tornado Science, in a Land With Plenty of Experience
Written by Brianna Blake
22 May 2006
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VOICE ONE:
This is SCIENCE IN THE NEWS in VOA Special English. I'm Bob Doughty.
VOICE TWO:
And I'm Faith Lapidus. This week the science of tornadoes.
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VOICE ONE:
For many parts of the United States, the spring season brings with it
tornadoes.
A tornado is a violently turning tube of air suspended from a thick cloud.
It extends from a thunderstorm in the sky down to the ground. The shape
is like a funnel: wide at the top, narrower at the bottom.
Tornadoes form when winds blowing in different directions meet in the
clouds and begin to turn in circles. Warm air rising from below causes the
wind tube to reach toward the ground.
Because of their circular movement, these severe wind storms are also known as twisters.
The most severe tornadoes can reach wind speeds of three hundred twenty kilometers an hour or
more. In some cases, damage paths can stretch more than one kilometer wide and eighty kilometers
long.
VOICE TWO:
With a tornado, bigger does not necessarily mean stronger. Large tornadoes can be very weak. And
some of the smallest tornadoes can be the most damaging. But no matter what the size, tornado winds
are the strongest on Earth. Tornadoes have been known to carry homes, cars and trees from one place
to another. And they can also destroy anything in their path.
Tornadoes have been observed on every continent except Antarctica.
But weather scientists say the country where they are most common is the United States. The United
States has more than one thousand tornadoes a year.
Last year, twelve of them resulted in deaths. In all, thirty-eight people were killed.

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VOICE ONE:
Tornadoes are observed most often in the central part of the
United States where the land is mostly flat. The area where the
most violent tornadoes usually happen is known as “Tornado
Alley.” This area is considered to extend from north central
Texas to North Dakota.
Weather scientists say Texas is the state with the greatest
number of tornadoes. But the state with the most tornadoes in
relation to area is Florida. In Florida, tornadoes often develop
along the edges of severe ocean storms.
Tornadoes can happen any time of the year. But they happen
most often in March, April and May. There is a second high season in November.
VOICE TWO:
In the spring, warm air moves north and mixes with cold air remaining from winter. In November, the
opposite happens. Cold weather moves south and combines with the last of the warm air from
summer.
Tornadoes can strike with little or no warning. Most injuries happen when flying objects hit people.
Experts say the best place to be is in a small room, without windows, in the middle of the lowest part
of a building.
People driving during a tornado are told to find low ground and lie flat, facedown, with their hands
covering their head.
People in the path of a tornado often have just minutes to make life-or-death decisions.
VOICE ONE:
The deadliest United States tornado on record is the Tri-State Tornado of March eighteenth, nineteen
twenty-five. It tore across Missouri, Illinois and Indiana. About seven hundred people were killed.
A "tornado outbreak" is often defined as six or more tornadoes produced by the same weather system
within a day. But the outbreak of April third and fourth, nineteen seventy-four, set a national record.
It is remembered as the "Super Outbreak."
One hundred forty-eight tornadoes struck during a twenty-four-hour period. More than three hundred

people were killed and six thousand others were injured.
One tornado that was especially destructive hit Xenia, Ohio. The sound you are about to hear comes
from the Web site ohiohistory.org.
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VOICE TWO:
No two tornadoes look exactly alike. And no two tornadoes act the same way.
A heavily damaged area of Iowa City, Iowa,
after a tornado in April
It takes the right combination of wind, temperature, pressure and humidity to create even a weak
tornado. Weather scientists can identify these conditions. And, when they observe them, they can
advise people that tornadoes might develop. But they are not able to tell exactly where or when a
tornado will hit.
Usually a community will receive a warning at least a few minutes before a tornado strikes. But each
year there are some surprises where tornadoes develop when they are least expected.
VOICE ONE:
The tornado reporting system involves watches and warnings. When people are told that a tornado
watch is in effect, that means tornadoes are possible in the area. A tornado warning means that a
tornado has been seen. People are told to take shelter immediately.
Yet tornadoes can be difficult to see. Sometimes only the objects they are carrying through the air can
be seen. Some nighttime tornadoes have been observed because of lightning strikes nearby. But
tornadoes at night are usually impossible to see.
Tornadoes that form over water are called waterspouts. But tornadoes cover a much smaller area than
hurricanes, which form over oceans.
Tornadoes can be measured using wind speed information from Doppler radar systems. Tornadoes
usually travel in a northeasterly direction with a speed of thirty-two to sixty-four kilometers per hour.
But they have been reported to move in other directions and as fast as one hundred seventeen
kilometers an hour.
VOICE TWO:
In the United States, the force of a tornado is judged by the damage to structures. Scientists inspect
the damage before they estimate the severity of a tornado. They measure tornadoes on the Fujita

scale. Ted Fujita was a University of Chicago weather expert who developed this system in the
nineteen seventies.
There are six levels on the Fujita scale. Tornadoes that cause only light damage are called an F-zero.
Those with the highest winds that destroy well-built homes and throw vehicles more than one hundred
meters are called an F-five.
VOICE ONE:
Some people make a sport out of watching and following tornadoes. They are called tornado chasers
or storm chasers. Their work can be seen in the extreme weather videos that are increasingly popular
on television.
Some chasers are part of weather research teams. Others do it to help document storms and warn the
public. Still others do it just because it is their idea of fun.
Storm chasers usually drive large vehicles to areas of severe weather. They follow storms for long
distances. For some, the appeal of a tornado is to get closer and take better pictures than others have,
without getting killed in the process.
VOICE TWO:
The National Weather Service says the United States gets more severe weather than any other
country. For one thing, it is also bigger than most other countries. And it has many different
conditions that create many different kinds of weather.

There are beaches and deserts, flatlands and mountains. The West Coast is along the Pacific Ocean,
which is relatively calm. The East Coast is along the Atlantic Ocean, which is known for its
hurricanes. These strike mainly the Southeastern states.
The hurricane season officially begins on June first and ends on November thirtieth.
(THEME)
VOICE ONE:
SCIENCE IN THE NEWS was written by Brianna Blake. Cynthia Kirk was our producer. I'm Bob
Doughty.
VOICE TWO:
And I'm Faith Lapidus. Read and listen to our programs at voaspecialenglish.com. And if you have a
science question, send it to We might be able to answer it on our show. Join

us again next week for more news about science in Special English on the Voice of America.